1. Field of the Invention
The present invention relates generally to wireless communication systems having a plurality of base stations and a mobile station, and in particular to the cell search process performed by the mobile station. The present invention is suitable for use in spread spectrum systems, and in particular in wide band code division multiple access (W-CDMA) systems, and it will be convenient to describe the invention in relation to that exemplary, non-limiting application.
2. Description of the Related Art
In W-CDMA and like wireless communication systems, a mobile station can receive a number of radio signals transmitted from several base stations, each radio signal serving a separate cell within a service area, via multiple propagation paths. A cell search process is carried out by the mobile station to synchronize the mobile station with a base station of a cell prior to transmission of traffic data, such as voice data.
A standard cell search process is shown in FIG. 1 and includes a slot synchronization step 1, a code-group identification and frame synchronization step 2, and long scrambling code identification step 3. During slot synchronization, the mobile station uses a primary synchronization code continuously transmitted in a primary synchronization channel to acquire slot synchronization to a cell. Slot synchronization is typically performed with a single matched filter or similar device matched to the primary synchronization code which is common to all cells. The slot timing of the cell can be obtained by detecting peaks in the matched filter output.
During the second step of the cell search procedure, the mobile station uses the synchronization channel's secondary synchronization code to find frame synchronization and identify the code group of the cell found in the slot synchronization step. This is done by correlating the received signal with all possible secondary synchronization code sequences, and identifying a maximum correlation value. Since the cyclic shifts of the sequences are unique, the code group as well as the frame synchronization is determined.
During the third step of the cell search procedure, the mobile station determines the exact primary scrambling code used by the found cell. The primary scrambling code is typically identified through symbol-by-symbol correlation over the Common Pilot Channel (CPICH) with all codes within the code group identified in the second step.
Having now identified the primary scrambling code used by the found cell, the identified primary scrambling code is compared in step 4 with a list 5 of previously identified scrambling codes in order to remove cells that have already been identified via the standard cell search process. Only those candidate cells for which the identified primary scrambling code is not found in the list 5 are then handled in subsequent post-processing operations.
When a mobile station performs a continuous cell search for the purpose of identifying new cells as they appear, the standard algorithm is to handle peaks detected in the first step of the cell search by comparing the timing of the peaks with peaks acquired in the previous run of the first cell search step. New peaks that have timing “very close” to the old peaks detected in previous runs of the cell search process are considered to be identical, and are rejected from further processing. Whether peaks are “very close” or not is determined by a dynamically adjusted parameter defining a maximum possible timing difference for which the peaks can are considered identical. The value of this parameter is derived from the maximum derived drift due to automatic frequency control (AFC) error and the moving path timing and current parameters of compressed mode.
The above-described standard cell search procedure dose not perform well when the first step is run within compressed mode gaps, that is when the peaks are far apart from each other, and also in idle mode when the paging occasions are frequently far apart. In this situation, the peak timing positions from subsequent first step runs frequently differ by a large amount. Accordingly, there is no information to identify which first step peaks belong to previously identified cells. The worst case outcome is that a first step peak belonging to the same cell is analyzed repeatedly with second and third steps because it keeps drifting out of the pre-set window. In such a scenario, the peak is repeatedly rejected after the third step as a multi-path signal and a new cell may therefore never be identified.